In the field of infrared detectors, there is a known technique of using devices arranged in the form of an array, and capable of operating at ambient temperature, in other words not needing to be cooled to very low temperatures, contrary to the imaging devices known as “quantum detectors”, which, for their part, require an operation at very low temperature, and typically that of liquid nitrogen.
These uncooled detectors traditionally use the variation in a physical quantity of an appropriate material, as a function of the temperature, in the vicinity of 300K. In the case of bolometric detectors, this physical quantity is the electrical resistivity.
An uncooled detector of this kind generally associates:                means for absorbing the infrared radiation and converting it into heat,        means for thermally insulating the detector, so as to enable it to heat up under the effect of the infrared radiation,        thermometry means which, in the context of a bolometric detector, implement a resistive element, formed from electrodes and a sensitive, so-called bolometric, material,        means for reading the electrical signals supplied by the thermometry means.        
The radiation absorption and thermometry means are integrated into a membrane suspended by the thermal insulation means above a substrate in which are placed the read means.
Detectors intended for infrared imaging are conventionally produced in the form of an array of elementary detectors in accordance with one or two dimensions, said array being formed in the manner known as “monolithic” or carried over to the substrate, generally made of silicon, in which are constituted means for the sequential addressing of the elementary detectors, and means for the electrical excitation and pre-processing of the electrical signals formed from said elementary detectors. Said substrate and the integrated means are commonly denoted by the term “read circuit”.
To obtain a scene via said detector, the scene is projected through a suitable optic onto the elementary detector array, and timed electric stimuli are applied via the read circuit to each of the elementary detectors, or to each row of such detectors, in order to obtain an electric signal constituting the image of the temperature reached by each of said elementary detectors. Said signal is processed in a more or less elaborate way by the read circuit, and then possibly by an electronic device external to the casing in order to generate the thermal image of the scene observed.
The main difficulty in employing bolometric detectors lies in the very slight relative variation of their electrical resistance, representing local variations in temperature of an observed scene, relative to the mean value of these resistances.
Indeed, the physical laws of thermal emission in the infrared of the observed scene, and typically between 8 and 14 micrometers (corresponding to the transparency band of the Earth's atmosphere in which bolometric detectors are generally used), lead to a differential power dP in the detector focal plane of the order of 50 μW/cm2 when the scene temperature varies by 1K around 300K. The determination of this value is easily within the scope of the person skilled in the art, by applying the aforementioned physical laws.
This estimation is valid for an optic with an aperture of f/1, a high transmission between the scene and the detector, and when the detector receives only a negligible quantity of energy outside the band of specified wavelength, for example and typically if the casing is fitted with a window that is transparent in this interval and opaque below and beyond the limits indicated.
As a corollary, the temperature variation dθ of a bolometer in thermal equilibrium, related to an infrared power dP absorbed on its surface S is given by the following expression:dθ=Rth·dP  (1)where Rth, is a thermal resistance between the sensitive part of the bolometer, which heats up under the effect of the infrared radiation, and the isothermal substrate which carries it.
Thus, for a bolometer with typical dimensions of the order of 30 μm×30 μm, which represents a surface of 9.10−6 cm2, the typical thermal resistance as set forth in the prior art, is of the order of 20 to 60 MK/W, which causes the bolometer to heat up by about 0.01 K to 0.03 K when the temperature of the scene element seen by the bolometer varies by 1 K.
If Rb denotes the electrical resistance seen between the two current lead poles in the sensitive bolometric material, the resulting resistance variation dRb is expressed by the following expression:dRb=Rb·TCR·dθ  (2)where TCR is a coefficient of relative resistance variation of the material constituting the sensitive part of the bolometer in the vicinity of the operating temperature, conventionally close to −2% per K for the usual materials in this field (vanadium oxides, amorphous silicon). As a consequence, the relative resistance variation dR/R consequential upon a differential of 1 K on the scene is of the order of 0.04%, i.e. 4.10−4/K.
Yet, the requirement today is for thermal resolutions of imaging much better than 1 K, and typically 0.05 K, or even less. Such results may be obtained by developing structures that have very high thermal resistances Rth through the implementation of sophisticated techniques. However the need remains to measure minute relative resistance variations and typically, as previously indicated, of the order of a few 10−6, in order to resolve temperature variations in time and space of a few tens of milliKelvin.
To explain how difficult it is to exploit such a slight variation, a diagrammatic version has been shown in FIG. 1 of a read circuit of a resistive bolometer 12 of resistance Rb, subjected to an infrared radiation and connected at one of its terminals to a predetermined constant voltage. The read circuit includes an integrator 10 including:                an operational amplifier 14 whereof the noninverting input (+) is set to a predetermined constant voltage Vbus;        a capacitor 16, of predetermined capacity Cint, connected between the inverting input (−) of the amplifier 14 and the output thereof;        a reset switch 18 connected in parallel with the capacitor 16, and controllable by means of a “Reset” signal.        
The read circuit further includes:                a first read switch 20, controllable by means of a “Select” signal and connected to the inverting input (−) of the operational amplifier 14;        an MOS injection transistor 22, whereof the gate is set at a predetermined constant voltage Vpol, whereof the source is connected to the other terminal of the bolometer 12, and whereof the drain is connected to the other terminal of the first selection switch 20; and        an information processing unit 23, connected at the output of the operational amplifier 14, and determining as a function of the voltage Vout at the output thereof the variation in the resistance of the bolometer 12 induced by the infrared radiation received by the latter, and thereby said infrared radiation.        
At the start of a read cycle of the bolometer 12, the reset switch 18, which is closed subsequent to a discharge cycle of the capacitor 16, is opened by adjusting the “Reset” signal to an appropriate value. The first read switch 20, which is opened, is switched into the closed state by adjusting the “Select” signal. The current passing through the bolometer 12 is then integrated by the capacitor 16. When a predetermined integration time ΔTint has elapsed from the start of the read cycle, the first read switch 20 is switched into its opened state. The voltage Vout, at the output of this integrator, image of the resistance Rb of the bolometer, is then given by the expression:
                              V          out                =                                                            V                bolo                                            R                b                                      ×                                          Δ                ⁢                                                                  ⁢                                  T                  int                                                            C                int                                              +                      V            bus                                              (        3        )            where Vbolo is the polarization voltage at the terminals of the bolometer 12 controlled by the voltage Vpol, assuming, in the interests of simplification, that Rb varies little during the integration time Tint.
An array of N resistances (bolometers) could thus be read according to this principle by means of the simultaneous integration (using N integrators) or sequential integration (on an integrator placed at line end or column end, or even a single integrator for the array) of the currents coming from each resistance.
When the array so produced is illumined by the projection of an infrared scene, Vout, will show spatial variations (coming from each bolometer) representing the scene. It will be remembered that the voltage Vout, as previously expressed is very largely constituted by a constant part from one detector to the other, which is therefore of no advantage in terms of imaging.
Furthermore, through the existence of the thermal coupling between the substrate and the bolometer, the thermal variations sustained by the substrate are transferred to the bolometer. Since ordinary bolometers show very significant sensitivity to such variations, the result of this is that the output signal is disturbed by this parasitic background signal, which is detrimental to the quality of the detection of the infrared radiation.
The sum of these disturbances, contained in the signal Vout, is generally known as the “common mode” signal.
Lastly, to read a bolometer an electric current must be made to flow therein. In fact, the bolometer heats up by joule effect (this is generally known as “self-heating”) further adding thereto a current component causing interference on the wanted signal related to the scene. Said self-heating Δθ(t), a function of the time, may be determined from the following differential equation:
            C      th        ×                            ∂          Δ                ⁢                                  ⁢                  θ          ⁡                      (            t            )                                      ∂        t              =                    V        bolo        2                              R          b                ⁡                  (                      θ            ⁡                          (              t              )                                )                      -                  Δ        ⁢                                  ⁢                  θ          ⁡                      (            t            )                                      R        th            where Cth, is the heat capacity of the sensitive membrane.
For a very short integration time Tint, of the order of a few tens of microseconds after the application of the voltage Vpol at t=0, the heating may be considered as linear and given by the relation:
      Δ    ⁢                  ⁢    θ    =                    V        bolo        2                              C          th                ×                              R            b                    ⁡                      (                          θ              ⁡                              (                                  t                  =                  0                                )                                      )                                ⁢          T      int      
It is then clear that, for typical values of Cth, Rth, Vbolo and Tint, said temperature rise by self-heating typically reaches several degrees Kelvin. Thus, even very limited technological spatial variations of Cth or Rb, of the order of 1% for example, are conveyed by spatial temperature variations of each membrane at the end of the integration time of the order of 20 mK for an electrical heating Δθ of 2°, in other words of the same order as the heating caused by a scene temperature increase of 1 K.
These dispersions thus also come to disturb the representativity of the signal Vout relative to the variations in space and time of radiative power, alone representative of the scene observed, and which constitute the wanted signal.
To overcome these drawbacks, a so-called “reference” resistive structure has been proposed as described in the document “Performance of 320×240 Uncooled Bolometer-type Infrared Focal Plane Arrays” by Yutaka Tanake et al., Proc. SPIE, vol 5074.
The principle of a reference resistive structure is to associate, with the resistive bolometer 12 in FIG. 1, an identical second resistive bolometer, polarized and connected to the substrate in an identical way to the first bolometer. Said second bolometer is further arranged so as to be essentially insensitive to the flux coming from the scene, typically via an opaque metal membrane, or placed in an area not illumined by the scene. The first and second resistive bolometers are furthermore associated in such a way that the current passing through the second bolometer is subtracted from the current passing through the first bolometer and that it is this current difference which is used by the read circuit.
To distinguish between the functions of these two bolometers, the expression “imaging” bolometer will be used for the first bolometer, and the expression “reference” bolometer for the second bolometer, even if in some uses, in thermometry for example, it is not necessarily an image that is formed, but for example a temperature measurement.
A reference structure 24 is diagrammatically shown in FIG. 2, which repeats the elements in FIG. 1, with which a so-called “reference” circuit 24 is associated. The reference circuit 24 includes a reference bolometer 26, an MOS polarization transistor 28 and a second read switch 30, substantially identical to the imaging bolometer 12, the MOS injection transistor 22 and the first read switch 20 respectively.
The elements 26, 28 and 30 are moreover polarized and arranged in the same manner as the elements 12, 22 and 20, the only difference being that the reference bolometer 26 is for example provided with an opaque metal membrane 32 that protects it from the radiation coming from the scene or is placed in an area not illumined by the scene.
The reference resistive structure finally comprises a current mirror 34, whereof one input branch is connected to a terminal A of the second read switch 30, and whereof the other input branch is connected to a terminal B of the first read switch 20. Said current mirror 34 substantially reproduces the current i1 passing through the reference bolometer 26 at the terminal B.
The employment of current mirrors means that only one single reference structure is needed per line, all of said structures being placed in accordance with one reference “column” for one matrix detector. Current mirrors are structures known to the person skilled in the art. Generally speaking they allow a reference current to be copied in a remote structure, and in particular allow said reference current to be distributed in a multitude of circuitry elements, irrespective of the resistive load thereof.
Thus, the current i1 passing through the reference bolometer is substantially equal to the common mode current, and the reference bolometer is subject to the same thermal variations coming from the substrate as the imaging bolometer. The difference i2−i1 between the current i2 passing through the imaging bolometer and the current i1 passing through the reference bolometer is then substantially free from the disturbances constituted by the common mode current and the component related to the thermal variations of the substrate, at least so long as the substrate is essentially isothermal. This current difference i2−i1 therefore corresponds substantially to the current induced by the resistance variation of the imaging bolometer 12 on account of its being heated by the infrared radiation coining from the scene.
Conventionally, there are two layouts that use reference bolometers.
In a first layout shown in FIG. 3, a reference bolometer 26 is provided for each line in an array 50 of imaging bolometers and therefore supplies, via the current mirror 34, a so-called “reference” current for all the imaging bolometers in the line. The self-heating phenomena of said imaging bolometers 12 are thus compensated since the reference bolometer 26 is subject to the same polarization cycles as the imaging bolometers 12 in the associated line. On the other hand, providing a reference bolometer 26 for each line in the array of imaging bolometers generates an on-line spatial noise given the technological dispersions of the reference bolometers.
In a second layout shown in FIG. 4, a single reference bolometer 26 is provided for all the imaging bolometers in the array 50. The current coming therefrom is then copied by a set of current mirrors 34. The spatial noise generated by the technological dispersions is thus avoided. However, the thermal cycle of said single reference bolometer 26 is substantially different from that of the imaging bolometers 12. Indeed, unlike an imaging bolometer which is polarized when its line is being read, the reference bolometer 26 is polarized at each line reading. The thermal time constant of the reference bolometer Rth×Cth, of the order of a few milliseconds, does not allow it to return to its equilibrium temperature prior to each read (integration) cycle. As a result, the self-heating of the reference bolometer 26 differs substantially from the self-heating of the imaging bolometers 12, in such a way that the rejection of this component is of very poor quality.
It should be noted that the higher the level of insulation of the membranes of the imaging bolometers 12 the more pronounced are the self-heating phenomena, which is the case in top-of-the-range detectors, in respect of which efforts are made to maximize Rth in order to maximize sensitivity, which also leads to reducing Cth in order to maintain a thermal time constant that is compatible with imaging frame frequency standards. Because of this, the heating after each reading is greater for similar stimuli, and finally the residual temperature rise at the start of the next cycle is all the more sensitive.
The more sensitive the detector the trickier therefore the common mode current rejection, which in the end limits the improvements in performance that it is possible to make to these detectors.
Furthermore, a reference resistive structure is technically difficult to produce. Indeed, to obtain a satisfactory operation thereof, it is necessary for the metal membrane 26 protecting the reference bolometer to be totally impermeable to the flux coming from the scene, while being thermally insulated from the other elements in the structure in order to avoid any thermal disturbance on the reference bolometer. Such a membrane is difficult to design and to produce. Additionally, the substantial increase in complexity involved in developing such a membrane necessarily entails an additional cost, given additional manufacturing stages and therefore a non ideal production performance. The same is true when the reference bolometer is placed in an area not illumined by the scene since this compels the provision of sufficient space on the surface of the read circuit substrate, in addition generally to an opaque screen added to the inside of the casing. The overall result is a higher detector cost.
In fact for detectors not requiring great precision, the reference bolometer is conventionally replaced by a so-called “compensation” bolometer, in other words a bolometer that has no optical occultation membrane. Such a compensation bolometer is generally thermalized in the substrate, in other words it has no attachment arms or is partially or totally formed in direct contact with the substrate in order to create a thermal short-circuit between the compensation bolometer and the substrate. The compensation bolometer therefore sustains essentially the same heating as the substrate. The common mode rejection then essentially comprises only the rejection of the signal part corresponding to the temperature of the substrate transferred to the imaging bolometers. The result is a common mode rejection of limited quality.